1. Field of the Invention
The present invention pertains to power-driven air filtering units and in particular to such air filtering units which are portable, being easily movable from room to room.
2. Description of the Related Art
A wide variety of air filters are available for use today. The filters typically include a blower arrangement for circulating air, and a filter stage or perhaps several filter stages disposed in the air path so as to filter or otherwise purify air flowing therethrough. The air filters differ widely in their volume capacity, their efficiency of filtration, and their ease of portability.
Several different types of filtering stages are available for use in present day air filters. The air filter stage may have, for example, an electrostatic precipitator type of element which charges dust or other airborne contaminants and attracts the airborne contaminants to an electrically charged grid. In general, electrostatic air filter stages are expensive to fabricate and install, and they require relatively costly maintenance. In addition, there is a substantial electrical power requirement in addition to that needed for maintaining an air flow through the filter stage.
As a second general type of air filtering, one or more layers of porous media are disposed in an air flow path for "mechanically" filtering or trapping airborne particles contained in the air flow. In general, if a greater efficiency of filtering is desired, additional stages or filtering layers are added. This, however, multiplies the cost of the initial filtering stage and frequently results in a significant pressure drop across the filter, thus requiring a higher fan capacity for maintaining a desired air flow.
Users of air filters are frequently concerned about the amount of air treated by a filter stage in a given time period. Some air filters are available as a relatively small scale device, such as an air filter for an ash tray. Such filters cannot be scaled up to a larger size so as to accommodate a user who desires several changes of air volume in a room, for each hour of operation of the air filter. This concept of room air filtering is frequently expressed as a number of "room changes" of air per hour. For purposes of comparison, building codes frequently require that public restaurants and the like public buildings have at least one or two air changes per hour, meaning that a volume of air equal to that required to fill a room is completely removed and replaced by fresh air at least once or twice each hour that the room is open to the public. The present invention is particularly concerned with air filters capable of filtering the air in an entire room on the order of once every hour and perhaps several times per hour.
In a practical air filter system, much more is needed than simply providing sufficient fan capacity to "turn over" the air filling a room at the desired rate. For example, the efficiency of the filter media, over the life thereof must be considered. As air is flowed through a porous filter media, airborne particles and the like are trapped in the filter media, thus reducing the porosity thereof and increasing the resistance of the air flow through the filter stage.
Other types of air filtering media may be provided which react with dissolved chemicals suspended in the air. Such filters frequently operate by adsorbing the chemical contaminants by collecting those contaminants in condensed form on the media surface. The adsorbed contaminants have the potential for changing the surface properties of the air filter media and in particular, have the potential of changing the resistance to air flow through the media. While an air filter stage could be "oversized" so as to provide a minimal acceptable air conductance at the end of its useful life, the cost of the air filter media in the stage rises significantly as does the size of that filter stage.
In addition, special considerations must be given to particular types of air filter media. For example, activated charcoal is a popular type of air filter media in use today and is frequently utilized as a bed of charcoal particles through which an air flow is conducted. U.S. Pat. No. 3,191,362 discloses an electrostatic air purifier, having a cabinet within which a blower is mounted to draw a horizontal air flow through the cabinet, inducing the air flow over energized electrodes and through a bed of activated charcoal filter particles disposed in the cabinet.
At times, design concerns center around the threshold question of operating efficiency of the activated charcoal bed for a given passage of an air flow therethrough. If this threshold question cannot be adequately resolved, there is little point in considering reduced efficiency of the air filter unit over time.
Presently, there exists a need for a high volume portable room air filter having at least a minimum filtering efficiency for types of contaminants frequently encountered in everyday situations. Of particular interest is the availability of an effective portable air filter for use in beauty parlors, in smoke-filled offices, and in sick rooms of patients suffering from asthma or other respiratory diseases. However, in order to be practical in use, the air filter should be sufficiently small in size and readily portable so that it can be moved from room to room and so that it can also be easily moved within a room without significantly changing the operation thereof, or adversely affecting the comfort of occupants in the room, located proximate the air filter. Several arrangements of portable room air filters have been proposed, yet the above-stated need for further improvements still exists. Examples of conventional air filter apparatus include that disclosed in U.S. Pat. No. 1,196,169 in which a cabinet is provided for enclosing a container for crushed ice and salt. A blower mounted atop the cabinet or a fan mounted within the cabinet induces an air flow over the surface of the container, thus cooling and dehydrating the air flow through the unit.
U.S. Pat. No. 3,757,495 discloses a portable air purifier having an elongated generally upright cabinet with a series of blowers located at the bottom thereof, adjacent a pair of opposed inlets. Adjacent each inlet are two layers of different filtering materials. The first filter material removes particles from the air flow that are greater than ten microns in size. The second air filter stage, made of activated carbon, aluminum silicate or the like, further filters the air to remove particles less than ten microns in size. In addition, if the second filter stage comprises activated carbon, certain contaminants may be treated while passing through the filter. Both filter stages are generally planar in configuration, with air flow being induced normal to their major surfaces. Air flow exiting the filters is flowed through a labyrinthine path of a compartment containing ultraviolet light lamps to provide further air purification. Purified air is then discharged through the top of the cabinet.
In U.S. Pat. No. 3,812,370, a portable air filter is provided also having a generally upright, elongated configuration. A cage-mounted blower is installed at the bottom of the cabinet and induces air to flow in an upward direction through a series of vertically oriented folded fiber filter constructions. A fine fiber, small particle filter material is supported by overlying layers of a coarse woven supporting net or screening. Upwardly directed air flows across the interior surfaces of the folded filter media and, being confined against further upward flow, is redirected to pass through the filter media, exiting therefrom in a generally horizontal direction.